The major effort of the proposed research will be to investigate two biochemical steps relevant to Leigh's disease (LD); (1) the normal synthesis of thiamine triphosphate (TTP) by brain, and (2) the effect of body fluid extracts from LD patients on this synthesis. This direction is based on recent observations on the pathogenesis of LD. Brains from such patients obtained at autopsy, have normal total thiamine, but a deficiency of TTP. This observation is consistent with the histologic changes in LD brains which are almost identical to those associated with thiamine deficiency states. Although the enzyme which converts TTP to thiamine diphosphate (TDP) has been well-defined, the cerebral enzyme(s) for the reverse, synthetic, system has not been identified. Also consistent with this theory of the biochemical basis of LD, is the finding of a factor in body fluids of LD patients which limits net TTP production by brain extracts. This factor, referred to as an inhibitor, could either inhibit the synthesis of TTP, or LD as well as to better understand the role of thiamine in the central nervous system, it is essential to purify both the enzyme(s) synthesizing TTP, as well as those causing its breakdown, and to test the effect of the "inhibitor" in the body fluids of LD patients on the purified systems. Another theory has been used to explain LD. Many of the abnormalities have been attributed to a deficiency of pyruvate carboxylase (PC). Review of the literature however, demonstrates no bona fide patients with autopsy-proven LD and absent PC. (I.e. either LD was not demonstrated by autopsy, or the PC assay was suspect.) Conversely all our patients with LD have had normal PC activity, and two patients with absent PC did not have LD. A third proposed theory to explain LD is a deficiency of pyruvate dehydrogenase (PDH). There are patients with PDH deficiency who do not have LD, but we have not assayed a large number of LD fibroblasts for this enzyme to further elucidate this theory. Such studies will be done as part of the proposed research.